Onychomycosis (Greek mukēs, fungus) is a fungal infection of the nails; onycholysis (Greek lusis, loosening) is the separation or loosening of a nail from its bed; onychogryphosis (Gk grupōsis, hooking of the nails) is the thickening and curving sideways of a nail, usually of the big toe; onychophagia (Greek phagein, to eat) is the formal term for biting one's nails.